1. Field of the Invention
The present invention relates generally to an arrangement for the correction of quadrature phase errors normally introduced by a synchronous demodulator. More particularly, the subject invention pertains to a system and method for determining quadrature phase errors between the two quadrature channels of a synchronous demodulator and provides a digital technique for correcting detected phase errors, without the need for high data sampling rates.
2. Discussion of the Prior Art
In almost all prior art digital systems which use a synchronous demodulator, quadrature phase errors are introduced by the synchronous demodulator which generates In-Phase (I) and quadrature phase (Q) channel signals. In radar systems, a quadrature phase error is highly undesirable because of doppler processing and adaptive null forming.
In digital systems utilizing a synchronous demodulator, such as pulsed doppler and adaptive processing radar systems, it is desirable that any quadrature phase errors introduced by the synchronous demodulator be corrected. In a doppler radar system, quadrature phase errors can create doppler frequency target sidelobes. In an adaptive processing radar system, quadrature phase errors limit the amount of clutter and jamming interference cancellation which can be achieved. The present invention provides a technique for correcting these quadrature phase errors normally introduced by a synchronous demodulator without the need for high data sampling rates.
In a doppler radar system, a quadrature phase error typically creates a frequency echo that appears as a false target at some doppler frequency other than that of the target. This frequency is a function of the magnitude of the quadrature phase error and the original doppler frequency of the received signal. FIGS. 6, 7 and 8 illustrate this effect for a target having a doppler frequency of 369 Hz for a system having 0.5-deg., 5-deg. and 50-deg. quadrature phase errors, respectively. As can be seen from these Figures, a peak appears at the target frequency (369 Hz) and a high false target also appears at 282 Hz. This false target increases as the magnitude of the quadrature phase error increases. Thus, unless the quadrature phase error is eliminated or maintained at a sufficiently low level, false target reports commonly known as ghosts are generated.
In an adaptive processing radar system, quadrature phase errors can severely degrade system channel match, which limits the amount of clutter and jamming cancellation that can be achieved. Any channel equalization techniques that may be used, such as transversal filters or multi-taps incorporated into the adaptive processing architecture, can only correct the quadrature errors to a very limited degree. FIG. 9 illustrates the small improvement (i.e. 2-3 dB) that can be achieved with a 32 tap transversal filter for two channels mismatched with a 5-deg. quadrature phase error.
Digital sampling techniques, such as those proposed in A DIRECT QUADRATURE SAMPLING APPROACH, D. R. Miedaner et al., Adaptive Technology, Inc., Mar. 29, 1989, and Charles Radar of MIT Lincoln Lab, can be used to create in-phase and quadrature channels that are completely free of synchronous demodulator quadrature phase errors. The publication by Miedaner et al. presents a description of a digital sampling technique that derives quadrature signals directly without using synchronous demodulators. However, these techniques require A/D converters with very high sampling rates. For instance, if a 4 MHz IF bandwidth is used to obtain a 125 ft. range resolution, an A/D converter with a sampling rate of at least 16 MHz is required to eliminate aliasing. In addition, the dynamic range requirements of the radar system calls for an A/D converter with 12-14 bits. With present state of the art technology, 12-14 bits at 16 MHz results in a rather large electronic package, which makes it inappropriate for many applications.
The present invention provides an alternate approach that uses readily available A/D converters (e.g. 12 bits, sampled between 5 and 10 MHz), and consists of a digital correction technique which is able to measure and correct for quadrature phase errors to within 0.01 degrees.